执行sql update use c#

今天犯了个大错

    public static void ChangeGoodsCounts(int GoodsID, int changCounts)
        {

            int lastCount;
            using (SqlConnection conn = new SqlConnection(connStr))
            {
                conn.Open();
                using (SqlCommand cmd = conn.CreateCommand())
                {
                    cmd.CommandText = "select GoodsCounts from GoodsInf where GoodsID=" + GoodsID;
                    int counts=Convert.ToInt32(cmd.ExecuteScalar());
                    lastCount = counts + changCounts;

                    cmd.CommandText = "update GoodsInf set GoodsCounts='"+lastCount+"' where GoodsID=" + GoodsID;
                    cmd.ExecuteNonQuery();

                }

            }
        }

http://www.tup.tsinghua.edu.cn/Resource/tsyz/030095-01.txt